1. Field of the Invention
The present invention relates to image forming apparatuses, such as printers, copiers, facsimiles, and multifunction peripherals, and particularly to an image forming apparatus that carries control toner images of a plurality of colors.
2. Description of the Related Art
In recent years, image forming apparatuses using an electrophotographic image forming process have been required to be equipped with technology that allows production of higher-quality color images at higher speeds. To achieve this, maintaining color stability, uniformity of color density, and the like is a challenge to be addressed. There is a widely-used technique in which control toner images for control purposes are formed in a non-image region. In this technique, the densities and positions of the control toner images are detected and given as feedback for adjusting conditions of the image forming process and the like, and thus, image stability is maintained. There are some examples of configurations for forming such control toner images. Japanese Patent Laid-Open No. 6-149057 discloses an image forming apparatus that forms control toner images on respective photoconductive drums. Japanese Patent Laid-Open No. 2001-318539 discloses an image forming apparatus that forms color patches on a belt.
Additionally, Japanese Patent Laid-Open No. 2007-079069 discloses a configuration for improved image stability during continuous image formation. In this configuration, during continuous image formation on a plurality of recording materials, control toner images are formed on a belt member in each inter-sheet gap (non-image forming region) between adjacent recording materials. During this image formation, when toner of the control toner images adheres to a transfer member that comes into contact with the belt member, the transfer member becomes contaminated with the toner. To remove the contamination from the transfer member, a cleaning unit is provided. Japanese Patent Laid-Open No. 2007-079069 further discloses a configuration in which, in each inter-sheet gap, control toner images of a plurality of colors are formed at different positions in the axial direction of the transfer member and thus, adjustment of a plurality of image forming units can be made in a short time.
As described in Japanese Patent Laid-Open No. 2007-079069 and Japanese Patent Laid-Open No. 2007-78937, in an image forming apparatus that forms control toner images, toner of control toner images not transferred to recording materials adheres to the transfer member. Then, when the contaminated transfer member comes into contact with the backside of a recording material, backside contamination of the recording material occurs. To prevent this, it is necessary to attach a cleaning device to the transfer member.
Specifically, an electrostatic cleaning device is attached to the transfer member. The electrostatic cleaning device brings a voltage-applied conductive brush member into contact with the transfer member so as to electrostatically attract and collect toner from the transfer member. Here, even when images are continuously output at short intervals between recording materials, it is possible to remove toner from the transfer member.
Japanese Patent Laid-Open No. 2005-128180 discloses an image forming apparatus in which image forming units for yellow, magenta, cyan, and black are arranged along an intermediate transfer belt. Here, it is possible to execute switching between a full-color mode and a black monochrome mode. When the black monochrome mode is set, the image forming units for yellow, magenta, and cyan are separated from the intermediate transfer belt so that unnecessary wear and power consumption can be prevented.
In a conventional image forming apparatus that forms control toner images of a plurality of colors at different positions in the axial direction of a transfer member, when an image forming mode in which image formation is performed without use of some image forming units is executed, the image forming units that are not used do not form control toner images.
In this image forming apparatus, when this image forming mode continues for a long time (e.g., 5 minutes or more) and then, the image forming units that have not been used (for cyan, magenta, and yellow for example) form control toner images, the following problem can occur. That is, the backside of a recording material may be contaminated in a region corresponding to positions on the transfer member where control toner images are formed, in the rotational axis direction of the transfer member, by the image forming units (for cyan, magenta, and yellow) that have not been used.
This backside contamination of the recording material can be solved by replacing a conductive brush member of an electrostatic cleaning device with a new one. An analysis of the replaced conductive brush member confirmed that a capability of the conductive brush member to remove toner from the surface of the transfer member was degraded in a portion in which the control toner images formed by the image forming units that had not been used adhered.
Moreover, the analysis confirmed that, in the conductive brush member, a portion that regularly collected, from the surface of the transfer member, control toner images formed by the image forming unit that had been used had a higher capability to remove toner than that of the remaining portion of the conductive brush member.